Surface acoustic wave (SAW) devices are well known in the art. These devices have found use in a variety of applications, particularly in stabilizing oscillator circuits in radar systems. For example, Doppler type radars require a very low noise frequency source in order to enhance the detectability of weak signals in a noisy background. These frequency sources (excitors) must generate several frequencies in the gigahertz range and they must be able to switch between frequencies rapidly. Surface acoustic wave resonators are used in oscillator circuits to generate signals at frequencies between 100 MHz and 1 GHz.
The surface acoustic wave resonant typically consists of a piezoelectric crystal, such as quartz, on which are placed two distributed reflectors. The reflectors are configured on the surface to form a resonant cavity which contains surface acoustic waves generated by a interdigitated transducers (IDT). The resonant condition created is similar to the conventional microwave and optical Fabry-Perot cavities, with the surface acoustic wave reflectors having a relatively narrow bandwidth, on the order of 0.5 percent.
Surface acoustic wave resonators have been used to stabilize voltage controlled oscillators. Oscillator circuits of the type used in Doppler radars comprise a surface acoustic wave resonator, corresponding inductors, a phase shifter and an amplifier. In voltage controlled oscillators having a surface acoustic wave resonator, power is coupled out of the resonant circuit by means of a directional coupler of the type known in the art. The directional coupler is typically configured to couple power from the resonant circuit from an amplified output signal. The directional coupler both extracts a minimal amount of energy from the oscillator circuit in a controlled manner for amplification, and isolates the oscillator circuit. Isolation is critical to the establishment and maintenance of a high degree of stability. Isolation is achieved by minimizing the amount of energy coupled from a resonator and also by eliminating the effect that external circuitry may have thereon.
It would be advantageous to have a surface acoustic wave resonator for use with a voltage controlled oscillator capable of direct signal coupling to the output amplifier, thereby eliminating the need for a separate directional coupler. The present invention is drawn towards such a resonator.